As far as I know there is no well-defined checksum offset.
The checksum is simply set by modifying any byte (which
normally should be unused).
Etherboot has some unused bytes at the beginning of rom data
and always uses the same offset 6.

Ah, so you don't actually update the checksum but change some unused
byte to make the checksum stay the same, right?

For other roms which also don't use the byte at offset 6, this approach
will work, too. If they store code or vital data at that location,
we destroy that data, so it won't work.
The VGA bios roms have a sequence of several bytes of zero
starting at offset 6, so maybe this data is not important and
we may change the byte at offset 6, but that should be checked
before using this mechanism.

Hmm. So offset 6 is the last byte of initVector. If (and only if) you
happen to know that the jump instruction takes 3 bytes only it is save
to modify the unused 4th byte. Seems to be true for both vgabios and
etherboot/gPXE. We can't assume this in general, although it is quite
likely given that there hardly would be anything but a 16bit jump.

As long as the driver specifies the romfile name,
we get an implicitly defined behaviour: either the
rom matches and nothing special is done, or it doesn't
and the id(s) will be fixed.

So neither flag nor opt-in seems to be needed.

When following this argumentation the vendor id sanity check shouldn't
be there in the first place ;)

Note that romfile is a pci bus property, so it isn't fully under the
drivers control because it can be overridden from the command line for
every pci device.